The possibility for application of hyphal interference as an additional method for confirmation of the specificity of the Phomopsis heltrnthi isolated from Helianthus annuus has been examined. With this aim, inter-relations of 13 fungal isolates, members of the Diaporthe/Phomopsis complex, have been studied. Microscopic investigations of the demarcation zones between colonies of the same and different isolates of Phomapsis/DiaporÛLe, revealed the phenomenon of the hyphal interference between different isolates. The results obtained suggest that hyphal interference could serve as an additional parameter for a more reliable determination of fungal specificity.